(1) Field of the Invention
The invention relates to the fabrication of integrated circuit devices, and more particularly, to a method of both forming ROM cell isolation in the fabrication of integrated circuits.
(2) Description of the Prior Art
A read-only memory (ROM) is a nonvolatile memory in which information is permanently stored through the use of custom masks during fabrication. Mask ROM's with buried bit lines (flat cell) are the most popular types of read-only memory. Conventionally, the buried bit lines are formed by doping impurities into the substrate through a bit line mask. When the wafer is subjected to high temperature conditions in subsequent processing steps, the doping impurities within the buried bit lines will diffuse outward and toward one another causing punch-through to take place between adjacent bit lines. Therefore, the conventional process includes a cell punch through (CPT) implantation through a CPT mask to isolate the buried bit lines and prevent outdiffusion of impurities from the buried bit lines.
U.S. Pat. No. 5,846,865 to Sheng et al teaches a method of forming a flat-cell mask ROM device using bit lines formed by tungsten-filled trenches in order to prevent punch-through. U.S. Pat. No. 5,842,815 to Liaw discloses a self-aligned contact halo implant for an SRAM. U.S. Pat. No. 5,849,615 to Ahmad et al uses a blanket implant to form both LDD and halo regions at the same time for a DRAM. U.S. Pat. No. 5,716,862 to Ahmad et al discloses an N-type halo angled implant to prevent punch through. U.S. Pat. No. 4,649,629 to Miller et al shows a boron-doped halo implant for a ROM.